1. Field of the Invention
The present invention relates to a dielectric filter. More specifically, the present invention relates to a dielectric filter having a plurality of dielectric coaxial resonators integrally formed in a single dielectric block.
2. Description of the Background Art
Generally, in a dielectric filter having a plurality of dielectric resonators coupled to each other, when coupling between adjacent resonators is capacitive coupling, an attenuation pole is obtained in the lower frequency range of the pass band, and when the coupling between adjacent resonators is inductive coupling, an attenuation pole is obtained in the high frequency range of the pass band.
Conventionally, in order to obtain capacitive coupling, resonator holes having steps have been formed in a dielectric block, as shown in FIG. 1. In the appended figures, shadowed portions denote portions where the base material of the dielectric block appear, that is, portions which are not provided with a conductor.
Referring to FIG. 1, in a conventional dielectric filter having resonator holes with steps, two resonator holes 2a and 2b, for example, are formed piercing through a pair of opposing surfaces 1a and 1b of a dielectric block 1 having approximately a rectangular parallellepiped shape. Inner conductors 3 are formed on the inner surfaces of resonator holes 2a and 2b. A pair of input/output electrodes 5 are formed at prescribed portions on the outer surface of dielectric block 1. An outer conductor 4 is formed approximately over the entire outer surface, except the regions where the input/output electrodes 5 are formed.
At one apertured surface 1a (hereinafter referred to as an open end surface) of each of the resonator holes 2a and 2b, there is a portion not provided with the inner conductor 3 (hereinafter referred to as a non-conducting portion), so that the inner conductors 3 are isolated (not conducted) from the outer conductor 4. At the other apertured surface 1b (hereinafter referred to as a short-circuited surface), the inner conductors are short-circuited (conducted) with the outer conductor 4. Between the inner conductor 3 of each of the resonator holes 2a, 2b and the input/output electrode 5, an external coupling capacitance is generated, which external coupling capacitance provides an external coupling.
In each of the resonator holes 2a and 2b, a step 21 is provided near the center of the open end surface 1a and the short-circuited end surface 1b. The inner diameter of the resonator holes 2a and 2b from the open-end surface 1a to step 21 is made larger than the inner diameter of resonator holes 2a, 2b from the short-circuited end surface 1b to step 21. A portion having larger inner diameter on the side of the open end surface 1a and a portion having smaller inner diameter on the side of the short-circuited end surface 1b are formed coaxially. In the dielectric filter structured as described above, the coupling between two resonators formed in resonator holes 2a and 2b is capacitive coupling, and an attenuation pole is formed in the low frequency range of the pass band. By changing the ratio of the lengths of the portions having larger inner diameter and smaller inner diameter, changing the ratio of the inner diameters, and so on, of the resonator holes 2a and 2b, the degree of capacitive coupling (coupling strength) can be changed. In other words, pass band characteristics such as band width can be adjusted.
In order to obtain inductive coupling, a coupling trench 6 is formed on the outer surface of dielectric block 1, such as shown in FIG. 2. More specifically, coupling trenches 6 are formed on both major surfaces of dielectric block 1 between resonator holes 2a and 2b as shown in FIG. 2. Coupling trenches 6 extend parallel to the resonator holes 2a, 2b, from the open end surface 1a and terminate near the center between open end surface 1a and short-circuited end surface 1b. The outer conductor 4 is formed on the surface of each of the coupling trenches 6. Resonator holes 2a and 2b are formed to have constant inner diameter, and the step 21 such as shown in FIG. 1 is not provided. Except these points, a dielectric filter has the similar structure to that shown in FIG. 1, and description thereof is not repeated.
In the dielectric filter shown in FIG. 2, the coupling between two resonators formed in the resonator holes 2a and an 2b is inductive coupling, and attenuation pole is formed in the high frequency range of the pass band. By changing the length, width, depth, position, cross sectional shape or the like of the coupling trench 6, the coupling strength of the inductive coupling can be changed. In other words, pass band characteristics such as band width can be adjusted.
In order to obtain inductive coupling, a step or a slit has been formed on the dielectric block in place of the coupling trenches 6 described above.
When attenuation poles are to be obtained in both the low frequency range and the high frequency range of the pass band, three or more resonator holes are formed in the dielectric block, a resonator having a step is formed in order to obtain an attenuation pole in the low frequency range, and a coupling trench or the like is formed on the outer surface of the dielectric block in order to obtain an attenuation pole in the high frequency range, and thus a dielectric filter is formed.
However, in the conventional dielectric filter having resonator holes 2a and 2b with steps 21 shown in FIG. 1, the coupling between the resonators is capacitive coupling, and it was difficult to obtain inductive coupling. Further, in order to change the coupling strength, that is, filter characteristics such as bandwidth, troublesome and complicated settings have been necessary, including adjustment of the ratio of the lengths of the larger diameter portion and the smaller diameter portion, and the ratio of inner diameters of these portions, of the resonator holes 2a and 2b.
In the dielectric filter having coupling trench 6 or the like formed on the outer surface of dielectric block 1 such as shown in FIG. 2, the outer shape of the dielectric block 1 is complicated, and therefore mounting on a substrate has been troublesome. In order to change the coupling strength, it is necessary to change the dimension, shape or the like of the coupling trench, step or the like, that is, it is necessary to change the outer shape of the dielectric block 1. More specifically, when dielectric filters having different characteristics such as different bandwidths are required, a number of dielectric blocks having different outer shapes corresponding to the required characteristics are necessary, and therefore standardization of the dielectric block is difficult. Further, reduction in size of the dielectric filter shown in FIG. 2 is more difficult than the dielectric filter shown in FIG. 1 having a step in the resonator hole of comparable characteristics, because of limitations in shaping the dielectric block.